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Mesosuchus
Mesosuchus is an extinct genus of rhynchosaur from the Early Triassic of South Africa. The type species is M. browni. It is known from the holotype SAM 5882, a partial skeleton, and from the paratypes SAM 6046, SAM 6536, SAM 7416 and SAM 7701 from the Aliwal North Euparkeria site. Mesosuchus is quite small, spanning around 30 cm in length. Mesosuchus was discovered and named by David Meredith Seares Watson in 1912. Fossil discovery SAM 5882, the holotype for Mesosuchus, consists of a partial rostrum, palate, braincase, lower jaws, sections of articulated presacral vertebral column, nine articulated caudal vertebrae, portions of scapula and pelvic girdle, and partial forelimb and hindlimbs.2 SAM 6046, one of the paratypes of Mesosuchus, consists of an incomplete right maxilla, an articulated series of the last ten presacrals, both sacrals, and first six caudals, partial forelimbs, left and right pelvic girdles, right hind limb, as well as element of left tarsus.2 SAM 6536, another paratype, consists of a virtually complete skull with lower jaws, articulated cervical vertebrae and ribs, dorsal vertebrae and ribs, complete left scapulocoracoid and partial right scapula, interclavicle, clavicles, distal end of left humerus, and gastralia.2 SAM 7416, another paratype, consists of an articulated vertebral column composed of the last dozen presacrals, both sacrals and at least the first 15 caudal vertebrae, fragments of right forelimb, pevic girdle, complete right femur, right crus and partial left crus, and right and left tarsi and pedes. Description Diagnosis All diagnostic traits of Mesosuchus come from diagnosis of the Mesosuchus browni as it is the only known species. Most of the autapomorphies used to identify Mesosuchus browni pertain mostly to the skull as that is the most researched since there are more parts for it to compare with others. The Mesosuchus is a small Rhynchosaurian diapsid with multiple rows of maxillary and dentary teeth with each row consisting of only a small number of teeth. The two premaxillary teeth are approximately twice the size of the maxillary teeth and the maxillary teeth inset medially while projected below the internal naris. There is also an occlusion between the vomerine teeth and dentary teeth with saddle-shaped vomers that overhang dorsally the premaxillary symphysis. Looking at the spine, the length of the axis neural spine is greater than the length of axis centrum. It also has a craniocaudally narrow neural spine of the third cervical and a prominent midventral groove on the first two caudal centra. Skull The Mesosuchus skull has a broadly triangular shape with a wide temporal region that tapers sharply along the orbits, expands abruptly at the prefrontals, and then tapers to the blunt rostrum. There is a large, median external naris located at the front of the rostrum that faces dorsally and cranially. The orbits face laterally and slightly cranially. One feature on the skull that can distinguish the Mesosuchus from all other Rhynchosaurs, with possible exception of Howesia, would be the presence of a beak-like rostrum that is formed primarily by huge premaxillary teeth rather than by tapering, edentulous premaxillae. Looking at the palate, it has a pronounced, sagittally elongated vault that is formed primarily by the vomers. The vomers contact the premaxillae at a level with the maxillary tooth margin towards the tip of the rostrum. However, it does curve strongly dorsally towards the palatines, and so we see the choanae very far above the tooth margin. To withdraw the choana further from the oral cavity, the vomerine and palatine borders of the choana are recessed dorsally. On the maxilla, there is a distinct notch that contributes most of the border of a dorsal fenestra. It appeared to be the cranialmost of a series of maxillary foramina that extended across the lateral face of the maxilla. This most likely hinted to cutaneous blood vessels and nerves in that area. This notch is hidden by an overlap of the premaxilla by the maxilla. The postorbital is composed of the anteroventral, posterodorsal, and dorsomedial processes in equal length. The anteroventral process does not come into contact with the ectopterygoid but it does indeed reach farther down the medial side of the jugal. The posterodorsal process forms a great portion of the smooth cranial rim of the upper temporal fenestra and contacts the parietal below the postfrontal. Information of the lower jaw had to be put together from the pieces of the different SAM fossils as there is not one that had a complete lower jaw. When looking at the different parts however, there are indications of at least six elements present in the lower jaw of the Mesosuchus: dentary, splenial, surangular, angular, prearticular, and articular. Dentition Evident on SAM 5882, 6046, and 6536 was a clear line of separation between the tooth and surrounding bone indicating that the teeth are implanted in the jaw, which went against the previous idea that the Mesosuchus marginal teeth were ankylosed to the jaws. However, although there are indications of deep implantation, it is still uncertain whether the mode of attachment would be described as thecodont. Looking at the maxillary teeth, a circular cross-section is maintained throughout the teeth and the majority are worn out heavily to short, blunt pegs.34 This suggests that Mesosuchus is in fact an herbivore. In 1963, M.E. Malan observed an interesting pattern in the positioning of the maxillary and dentary teeth. The middle section, where the medial expansion of the maxilla is wideset, had a zig-zag arrangement of teeth while the first and last four teeth are aligned in a row that is parallel to the maxilla. In the dentary, a simpler arrangement with only a slight zig-zag arrangement.35 Comparing this feature to the Captorhinus aguti who also had zig-zag pattern of teeth, they hypothesized that Mesosuchus could have multiple teeth on at least the maxilla. This hypothesis was supported when examining the erosional features on SAM 6536. Discovery & Etymology Bones of Mesosuchus were first found by David Meredith Seares Watson in 1912 after examining a block of sandstone kept in a private collection of Mr Alfred Brown.6 This block was found in the middle deposits of the Burgersdorp Formation7 in the Cynognathus Assemblage Zone near the town of Aliwal North in the Cape Province of South Africa.3 The block of sandstone contained many intermingled partial skeletons of several small reptiles and after careful sorting, Watson considered the unidentified skeletons to belong to a single new species which he named Mesosuchus browni. As the etymology of the name suggests, Watson believed that Mesosuchus was an ancestral crocodile with close affinities to other presumed primitive crocodilians such as Proterosuchus, Erythrosuchus, and Ornithosuchus. However, in 1913, Robert Broom looked more closely at the partial skeletons and immediately determined it to be in fact skeletons of two distinct, thought related, species. Broom designated an articulated skeleton with a single external naris and a pair of supposed acrodont premaxillary teeth as the type of Mesosuchus,6 and the remainder of the specimens were assigned to a new genus and species Euparkeria capensis. Looking back, we see the confusion Watson had when giving the name Mesosuchus as the data for his opinion were derived from the skeletons of Euparkeria, in particular the slender lower jaw with thecodont implantation, the crocodilian-like ilium, and the construction of the tarsus and pes. References * In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods by Nicholas C. Fraser and Hans-Dieter Sues Category:Archosaurs Category:Prehistoric reptiles of Africa Category:Triassic reptiles Category:Rhynchosaurs